A flat disk or "wafer" of single crystal silicon is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Semiconductor wafers are typically formed by growing an elongated cylinder or ingot of single crystal silicon and then slicing individual wafers from the cylinder. Multiple layers of conductive material and dielectric material are thereafter built up on the wafer in order to form a multilevel integrated circuit.
The front face of the wafer on which integrated circuitry is to be constructed must be extremely flat in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. The removal of projections and other imperfections is referred to in the art as planarization. Material layers applied to the wafer as integrated circuitry is built must also be planarized in order to produce extremely flat surfaces free of irregularities or projections. To this end, chemical mechanical polishing ("CMP") machines have been developed, and are well known in the art, to provide controlled planarization of semiconductor wafers and layers deposited thereon.
CMP machines generally include one or more wafer carriers or "chucks" which retain and carry wafers to be planarized and which press the front faces of the wafers against the surface of a rotating polishing pad. The wafer carrier is also typically rotated to effect relative lateral motion between the polishing pad and wear and planarization of the wafer face due to frictional contact against the pad. An abrasive slurry, such as a colloidal silica slurry, is usually introduced at the pad-wafer interface in order to augment the planarization process.
A typical wafer carrier includes a rigid pressure plate and a flexible backing pad secured thereto. The rear face of the wafer is mounted against the backing pad, while the front face of the wafer is exposed to the polishing pad. The backing pad serves several important functions. It cushions the wafer and protects it against damage which may result from direct contact with the rigid pressure plate. Moreover, as downward pressure is applied by the pressure plate to press the wafer against the polishing pad, imperfections or asperities present on the rear face of the wafer are "telegraphed" through the wafer to its front face, resulting in uneven pressure distribution across the wafer front face against the pad which, in turn, leads to uneven material removal rates and impaired planarization. The backing pad acts to absorb any imperfections or asperities present on the rear face of the wafer to prevent uneven pressure distributions and corruption of the planarization process from occurring. Finally, the pad frictionally engages the rear surface of the wafer, thereby preventing movement or sliding of the wafer relative to the backing pad.
Maintenance of a uniform and consistent pad profile or shape is critical to achieving uniform wear across the wafer as it is being polished. Inconsistencies, nonuniformities and deformations in the pad are telegraphed to the front face of the wafer in the same fashion that asperities on the rear face are telegraphed. Many known backing pads are inadequate in this regard as they are formed from materials, such as urethane elastomers, that are characterized by behavior that is plastic as well as elastic. U.S. Pat. No. 4,319,432 to Day, for example, discloses use of urethane backing pads. U.S. Pat. No. 4,811,522 to Gill, Jr. discloses use of a porometric film deformable to such an extent that it is subject to a 40 to 60 percent reduction in its original thickness. Contact adhesives used to bond the pads to the carrier further complicate the plastic behavior as they also move and deform over time. The plasticity of the pads and the adhesive layers leads to permanent strain or deformation of the pad under repeated shear and compressive loads. High stress applications, such as the polishing of tungsten layers applied to wafers, causes even more rapid and serious deterioration of wear uniformity due to plastic deformation of the backing pad. Resins such as urethane are also hydrophillic and their properties can change over time and with chemical exposure.
Backing pads of porous materials are also frequently utilized. Examples of such pads abound in the art and may be found in U.S. Pat. No. 3,841,031 to Walsh; U.S. Pat. No. 4,258,508 to Wilson et al.; U.S. Pat. No. 4,519,168 to Cesna; U.S. Pat. Nos. 5,101,602 and 5,157,877 to Hashimoto; and U.S. Pat. No. 5,538,465 to Netsu et al. These pads have been problematic in that they often become loaded with abrasive buildup from the slurry. As the pad is repeatedly used, its profile changes due to the presence and action of the abrasive. This also results in nonuniform wear patterns on the wafers that become progressively worse as the pad profile continues to change.
U.S. Pat. No. 4,132,037 to Bonora and U.S. Pat. No. 5,335,457 each mention the possibility of using a backing pad formed of silicone rubber. Though alleviating plastic deformation, silicone rubbers have been found to be not suitable in backing pad applications as they are extremely slippery when wet and coated with fine slurry particles and do not provide sufficient friction or surface adhesion between the wafer and pad. The wafer tends to move in the planar direction during polishing and non-uniform material removal rates result.
Many known backing pads are also secured to the pressure plate through use of a separate and deformable adhesive layer. The adhesive layer presents another opportunity for introduction of particles or other imperfections into the stack above the wafer which may impair planarization. U.S. Pat. No. 4,132,037 to Bonora, for example, uses transfer tape to secure the backing bad; U.S. Pat. No. 4,141,180 to Gill, Jr. et al. employs an adhesive; and U.S. Pat. No. 5,205,082 to Shendon et al. utilizes glue. Bonora, in addition to using an adhesive, uses a multi-layer backing pad, the layers of which are also secured together by adhesives. Use of adhesives is also problematic in that the adhesives tend to move and deform under load in a plastic fashion, thereby altering the profile of the pad.